Communications in a cave environment

Lava tubes are thought to exist on Mars and on the Moon, and are of special interest for science objectives and human habitability. The communications environment in such a cave is important for exploration. This paper explores the use of the shooting and bouncing rays (SBR) approximation to modeling the electromagnetics environment in a basalt rock lava tube with an obstruction, such as a cave-in. The cave, which has smooth walls, widens until the obstruction occurs, and then narrows. There is no line-of-sight communications path. A cave length of 100 meters is modeled. Existing knowledge of tunnel communications and the ANSYS Savant tool are leveraged in order to provide results for the cave geometry. A number of observations are offered in the following areas based on indications obtained from lava tube modeling: (1) Enhanced signals are observed. This is the degree to which the cave walls can create an amplification of the received signal and assist communications by reflecting the incident and re-scattered signals. Communications behind the obstacle is possible. (2) Spatial signal fading was observed, hinting at geometric effects or that scattered signals can combine destructively. Deep fades occur but appear to be rare. (3) Diminished signal strength occurs at the boundaries. Robotic exploration may take place close to the cave floor or walls. Indications from simulation for signal strengths close to the cave floor and walls are examined. (4) Frequency-selective fading can occur. The time-delay spread is discussed. The simulation results include an evaluation of IEEE 802.11 WiFi at 2.4 GHz using a freestanding electric cross dipole antenna and transmitted signal powers from 10mW to 100mW. Receive data rates are derived at locations throughout the cave and are provided to the degree that the cave geometry and electromagnetics approximations hold. These are only estimates of actual data rates subject to the approximations that were applied. Simplifications and adaptations of the WiFi waveform are discussed as part of a power trade.